The present invention relates to a gamma camera system for detecting gamma rays emitted from radioisotopes (RIs) injected to a subject to image a RI-distribution.
In the gamma camera system, there are two main types. One is a single photon nuclide, which emits one photon at RIs collapse time. The other is a positron nuclide, which inversely emits a pair of photons at a positron disappearing time. The following will explain the general former type.
The main detector of the conventional gamma camera system is an anger-type detector. The anger type detector comprises a collimator for limiting an incident direction of gamma rays, a scintillater for converting the gamma rays to light, a light guide for guiding light, a plurality of photoelectric multipliers (PMTS) for detecting guided light, and a lead shield for shielding disturbance gamma rays.
The anger type detector having a rectangular effective visual field of (35 cm.times.50 cm) has a size of 55 cm.times.70 cm.times.25 cm (thickness), and a weight of over 300 kg. Due to this, a stand and arms were inevitably increased in their sizes and weight to ensure support of such a big and heavy detector.
The gamma camera system has the following kinds of imaging methods:
(Statistic imaging method)
According to this imaging method, the gamma rays are continuously detected for a fixed period of time by the single detector fixed to the subject to obtain the RIs distribution (plane image).
(SPECT imaging method)
According to this imaging method, one detector is rotated around the subject. During the rotation of the detector, the counting of the gamma rays is repeated. Then, the RIs distribution (tomographic image) such as a CAT scan is reconstructed based on the obtained count values of the gamma rays.
(Two-detector opposite SPECT imaging method)
According to this imaging method, two detectors, which are arranged to be opposite to each other, are rotated around the subject. During the rotation of these detectors, the counting of the gamma rays is repeated. Then, a tomographic image such as a CAT scan is reconstructed based on the obtained count values of the gamma rays.
(Three-detector 90.degree. SPECT imaging method)
According to this imaging method, three detectors, which are arranged in a triangular form, are rotated around the subject. During the rotation of these detectors, the counting of the gamma rays is repeated. Then, a tomographic image such as a CAT scan is reconstructed based on the obtained count values of the gamma rays.
In these imaging methods, various kinds of corrections are needed to improve the image quality and the accuracy of a quantitive measurement. For example, there are an energy correction, a linearity correction, a uniformity correction, a scatter correction, a crosstalk correction, an absorption correction, etc. The linearity correction corrects distortion of the peripheral edge of the visual field. The uniformity correction uniforms variations of sensitivity of PMTs. The scatter correction removes a scatter component. The crosstalk correction corrects a crosstalk between two kinds of RIs of different peak energy. The absorption correction corrects a counting error caused by uneven absorption ratios of the organism.
To perform the absorption correction, it is necessary to measure the distribution of the absorption ratios of the subject by use of an emitter in which a radiation frequency of the gamma rays is uniform. The gamma rays emitted from the emitter are sent to the subject. Then, the detector the gamma rays are transmitted through the subject.
If the absorption ratios of the subject are spatially uniformed, the number of incidence of the gamma rays (count values) becomes the same value without depending on the detected position. However, in actuality, since the absorption ratios are not uniformed, the count values depend on the detected position and become uneven.
Therefore, the absorption ratios are obtained, and the count values are multiplied by a reciprocal number of the obtained ratios. As a result, the condition that the absorption ratios are fixed can be satisfied in view of approximation. This is the absorption ratio correction.
However, the absorption correction has the following problems.
(1) The anger-type detector has low energy resolution. The accuracy of the scatter correction or that of the crosstalk correction is reduced by the low energy resolution. The scatter correction with a low accuracy or the crosstalk correction with a low accuracy decreases the accuracy of the absorption correction.
(2) Since both the gamma rays for imaging and the gamma rays for measuring absorption ratios are detected by the anger-type detector, each detection must be operated by an operator. As a result, photographing time was considerably extended.
(3) Since the anger-type detector was so large and heavy, there was a limitation in approaching the detector to the subject. As a result, the measuring accuracy of the absorption ratios was low.
Moreover, the conventional gamma camera system has the following problem.
As mentioned above, since the anger-type detector is so large and heavy, the degree of freedom of the positioning is low. As a result, this kind of gamma camera system cannot correspond to various kinds of imaging, and the specialization of the gamma camera system has advanced.